A muffler or exhaust device, used in a straddle type vehicle (such as a motorcycle), has opposing design requirements. A muffler or exhaust device is required to effectively exhaust gas from an engine with high efficiency and simultaneously reduce or deaden the exhaust sound that is produced when highly pressurized or heated exhaust gas is discharged.
Currently, noise regulations are tightening, causing the requirements of sound reduction or sound deadening to increase. For this reason, it is desirable to achieve sound reduction or sound deadening while maintaining exhaust efficiency. One example of a muffler for a motorcycle that attempts to address the competing requirements of exhaust efficiency and sound deadening is described in Japanese Unexamined Utility Model Application JP-A-2007-231784.
When a muffler or exhaust system design is considered for its exhaust efficiency only, it is preferable to keep the muffler or exhaust system as straight as possible. However, when the muffler is extended straightly, the muffler cannot be fit into the vehicle body of a motorcycle. Thus, a muffler is brought to the rear of the vehicle body with as subtle of bends as possible in order to minimize exhaust resistance. Furthermore, the design of a straightly extended muffler is often difficult to achieve because of the connection of the front wheel and the consideration of the lean angle. Normally, a muffler ideal in length for the engine performance, does not fit the shape of a motorcycle without modification. Compared to designing a muffler for a four-wheeled motor vehicle, it is very troublesome to design a muffler for a motorcycle that has a length optimized for the best performance, yet still maintains the best possible smooth shape to fit the shape of the motorcycle.
Exhaust efficiency is not the only issue. In a motorcycle, muffler weight has a great impact on operability. Because a motorcycle is light in body weight, an increase in weight as small as 1 kg can have a significant effect. In addition, operability of the motorcycle is also adversely affected if the center of gravity of the muffler is located to far from the center of gravity of the motorcycle.
No matter how carefully the structure of the muffler is designed, a certain muffler capacity is still required in order to meet sound deadening requirements. A muffler usually has to be enlarged in order to conform to tightening noise regulations. Additionally, if the weight of the muffler is reduced by using a thinner metal plate during construction, the thinner metal plate will vibrate more and increase the noise. Thus, the muffler weight tends to be unavoidably heavy. The increased weight of the muffler deteriorates the operability of the motorcycle.
In this way, the structural design of muffler for a motorcycle is determined by various opposing factors. It is extremely difficult to design a muffler that is compact while still maintaining exhaust efficiency and sound-deadening characteristics.
U.S. Pat. No. 3,263,772 describes a high frequency silencing element (reference numeral 16 in FIG. 1 of the same publication) attached to a pipe (reference numeral 10 in the same FIG. 1) as an exhaust gas system for an automobile. However, this high frequency silencing element is not a resonator. Specifically, high frequency components are silenced by the air in a cavity (reference numeral 22 in the same FIG. 2) of the high frequency silencing element or porous fibrous materials.